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In ISON's Wake, a Trail of Questions

Submitted by Karl Battams on Fri, 11/29/2013 - 23:18

I always find that writing the first line in a blog post is the hardest, and this has never been truer that now as I struggle to decide where I should even begin. My @SungrazerComets Twitter feed, and my email accounts, are all blowing up with questions about comet ISON. Many of them have already been answered, and many of them have unsatisfying answers, but I'll do my best. First, a little personal note...

All of us in the CIOC have been overwhelmed by the positive feedback we have received from the public through both our website, and our emails and, for me, my Twitter feed. I truly wish I could respond to every message, but I simply can't, so instead say to every single one of you who have sent us messages of support, encouragement and thanks, or simply visited out site and read our thoughts and information... we THANK YOU! Truly and sincerely! Astronomy and science is our passion, and we are simply delighted and honored to have had such a broad and receptive audience to share that passion with. On the past couple of nights, as exhaustion has started to get the better of me, the supportive messages have definitely kept me going. [*sniffle*]

OK, back to more serious but still awesome stuff, and before I get to those questions I hinted at earlier, I should probably share with you a couple of new movies!

I'm sure you've all seen the LASCO movies by now - everyone is talking about them. But these movies are from STEREO and are pretty fresh, as we only just got back the high-resolution data. WARNING: These are big animated gif and might take a minute to load... please be patient as they're totally worth it!

Pretty cool, huh?! Now I'm sure you have even more questions, and I promise we're almost there but first I want to be sure you understand what you're looking at above.

These images are "coronagraph" images, just like the SOHO/LASCO images we've been seeing, and the Sun is represented by that white circle in the center. But the spacecraft that took these images are in very different locations in space -- on the opposite side of the Sun, in fact! I recommend you check out this image that will show you exactly where the spacecraft were relative to the comet when these movies were made. (Keep in mind also that the SOHO spacecraft is close to Earth, so we have three unique views of ISON, which is just fabulous!) I also want to quickly note that these images have been processed to make them look "pretty", and have been rescaled, colorized and compressed. Thus these should not be used for robust scientific analysis -- you will need the actual "FITS" image data files for that. These are just a very beautiful look at a truly spectacular comet!

OK, Q&A time...

What happened to comet ISON? Is it still alive?Great question, and we recommend you find a comet expert to answer that. ... [crickets] ... Sigh, OK I guess that's us. Well we don't have a very clear answer yet but there are a couple of things we can say for sure. First, during its passage through the Sun's million-degree corona, its dusty/gassy coma got very much burned away, though clearly some fine dust survived (which is the fine cloudy stuff you see being pushed away from the Sun). Second, something did emerge from the corona. It could be a comet, or just the remains of what once was. We can't tell right now.

Certainly there's lots of dust, and Matthew and I hesitantly lean towards thinking that there's something there producing dust, but that could be a small nucleus, or it could be a pile of rubble and comet chunks that will dissipate in the coming days. The key thing we need to find out -- and as yet we have no data about this as ISON is too faint and close to the Sun -- is whether there is any gas being produced. If there's gas, there's almost certainly an active nucleus; if there's almost no gas then probably no nucleus.

If there is a nucleus, how big is it? There is no way we can tell this from the spacecraft data we have right now. We will need to wait for Hubble to be able to observe the comet, which will be in mid-to-late December, I believe. What I can tell you is that however big ISON's nucleus was a few weeks ago, it is much smaller now!

Will it be naked eye visible? When? How bright? This is definitely the toughest question but also the most frequent. We still don't know if it will be naked eye but based on its current brightness in the LASCO images - which is around magnitude +5 and fading - it does seem unlikely that there will be much to see in the night sky. I suspect that some of the outstanding astrophotographers around the world will be able to get something, but I doubt it will be as spectacular as before perihelion. I hope I'm wrong though.

I'd guess that a few observers will begin picking up ISON in a couple of days but if - and I do mean IF - comet ISON becomes naked eye visible, it won't be until near the end of next week (say, Dec 6 or 7). Please don't get your hopes up, but we all need to keep in mind how ISON keeps surprising us.

Has ISON changed course? Is it now a danger to Earth? I'm still getting lots of questions about this, so it's important to address. It is also by far the most absolute and definite answer I can give. The answer is "NO" -- whatever is left of ISON, be it nucleus, rubble, or dust, -- the large chunk(s) will continue to follow the same orbit that we always said they would. The passage through the Sun, and any possible fragmentation, will not have made a difference. This means that ISON is absolutely NO THREAT to Earth in any way whatsoever. The comet, or its remains, will now pass harmlessly out into space, never to be seen again.

Folks also continue to be worried about a "meteor shower" at Earth so that's worth mentioning. As I understand it, Earth will probably pass through the remnant trail of comet ISON's tail some point early next year. The net result of this will be at least one or two, if not a handful, or extra shooting stars in the sky over a couple of nights. And that's it. Just shooting stars, just like we see every single night on Earth. Any dust that Earth encounters will be absolutely tiny and stands zero chance of reaching anywhere near the Earth's surface. Indeed, Earth passes through numerous comet tails every year -- that what the Leonids and Perseids are, for example. So don't worry about this, and don't even expect to notice it.

Congratulations: you now know as much as we do! No, seriously. We still have way more questions than answers, and it's going to take a while longer to get things figured. In terms of a timeline, in a few days time I think we'll be able to make a good call on the naked eye visibility of ISON (keep your expectations low, please...). In maybe a week or so I'm guessing we'll fully understand why ISON didn't put on a show in SDO (they might have already figured this out - I don't know - but that's their story to tell, not mine). Give us three or so weeks at least to (hopefully) get Hubble images to say if there's anything left of ISON, and how much, if so. Then expect results from the Comet ISON Observing Campaign to continue appearing for at least, oh, five years? Maybe ten...

And that last point is the one I'll end on tonight: over the past year, we've amassed what we believe to be the largest single cometary dataset in history from one of - if not the most - successful coordinated observing campaigns in history. That data is going to tell us a lot, but is going to take a seriously long time to sort through. We've had a crazy year, an even crazier past few months, and a truly insane couple of days. But everything we get out of this will make it more than worth it, and for me it's just a privilege to have played a part in this unprecedented and extraordinary event.
Keep up-to-date on the latest ISON and sungrazing comet news via my @SungrazerComets Twitter feed. All opinions stated on there, and in these blog posts, are my own, and not necessarily those of NASA or the Naval Research Lab.

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Image of the Week

Every week, a new image related to Comet ISON!

This week: The Final Nail in the Coffin? When ground-based observers came up empty-handed in the hunt for ISON's remains, we turned to the Hubble Telescope to see if its ultra-powerful instruments could find anything...Earlier picks can be found in the ISON Image of the Week Archives. Note that this feature will become monthly, instead of weekly, in 2014.